1. Field of Invention
In modern motor vehicles, systems which contribute to active or passive protection of the vehicle occupants are being increasingly used. Systems for monitoring the tire pressure protect the vehicle occupants against injury which would otherwise be caused, for example, by an abnormal tire pressure. An abnormal tire pressure can, for example, increase the wear of the tire and the consumption of fuel, or a tire defect (“flat tire”) may occur. Various tire pressure monitoring systems, which either operate on the basis of directly measuring sensors or detect an abnormal tire pressure by means of an evaluation of rotational speed properties or oscillation properties of the vehicle wheels, are already known.
2. Description of the Related Art
What are referred to as directly measuring tire pressure monitoring systems, which, by means of pressure sensors in the individual tires, sense the respective pressure in the respective wheel, are known, as described for example in application DE 199 26 616 C2. Furthermore, what are referred to as indirectly measuring tire pressure monitoring systems (DDS: Deflation Detection System), which can sense a pressure loss from auxiliary variables, for example by comparing the rolling circumferences of the individual wheels, are also known, for example from DE 100 58 140 A1.
WO 01/87647 A1 describes a tire pressure monitoring method and device which combine a tire pressure monitoring system which is based on the sensing of wheel radii and a tire pressure monitoring system which is based on the evaluation of oscillation properties.
Only relative changes in the rolling circumferences of up to three wheels compared to a learnt state can be detected with indirect tire pressure monitoring methods which are known from the prior art and which are based on the observation of rolling circumferences. The absolute rolling circumference of the tires is unknown in these indirect methods which are known from the prior art, since independent information about the vehicle speed is not available. For this reason, a simultaneous pressure loss at all four tires cannot be detected by these systems.
A method for determining the absolute rolling circumferences of the wheels of a motor vehicle from the wheel speed information is disclosed in DE 10 2005 014 099 A1. Here, the time difference between the occurrence of corresponding vibrations in the wheel speed signals at the front wheel and rear wheel of the same side of the vehicle is used to determine an absolute vehicle speed and the absolute rolling circumferences of the four wheels.